1. Field
The present disclosure relates generally to noise and, in particular, to reducing noise generated by a structure. Still more particularly, the present disclosure relates to a method and apparatus for reducing noise generated by composite and metallic structures.
2. Background
Traditionally, existing aircraft have been constructed of metallic structures. However, currently, aircraft are being designed and manufactured with greater and greater percentages of composite structures. Metallic structures are structures that are comprised of one type of metallic material. Composite structures are comprised of composite materials. A composite material is a material made from two or more materials with different physical and/or chemical properties that remain separate and distinct on a macroscopic level within the finished structure.
Some aircraft may have more than 50 percent of their primary structures made from composite materials. Composite materials are used in aircraft to decrease the weight of the aircraft. This decreased weight improves performance features, such as payload capacities and fuel efficiencies. Further, composite materials provide longer service life for various components in an aircraft.
The amount of noise present in an aircraft is a concern with respect to providing a pleasant travel experience and a desired level of comfort for passengers. For example, it is desirable to minimize the amount of noise in the interior of an aircraft, such as a cabin.
Although reductions in weight are obtained through the use of composite structures, these lighter weight composite structures have a higher stiffness as compared to metallic structures. This higher stiffness and reduced weight results in an increased amount of noise radiated from these composite structures.
Controlling the noise in the interior of aircraft with either or both composite and metallic structures poses a number of technical challenges. For example, the level of noise generated by composite structures may be determined by the resonant modes of vibration of the composite structures. These resonant modes of vibration pose technical challenges when trying to control the noise generated by the composite structures.
As another example, a metallic structure in an aircraft follows mass law. In the example of a metallic structure, mass law specifies that a reduction in noise generated by the metallic structure of about six decibels may be achieved by substantially doubling the mass of the metallic structure. This doubling of mass adds undesired weight to the aircraft. Further, in controlling noise, various factors, such as weight, cost, and system performance, are taken into account.
Currently, noise is controlled in the interior of the aircraft using a noise control system that has several elements. These elements may include, for example, damping tiles, fiberglass blankets, acoustic foam, trim panels, isolators, and/or other suitable elements. For example, a layer of damping tiles may be placed on and attached to the inside skin of an aircraft. These damping tiles may contain a viscoelastic layer configured to convert vibrations of the inside skin of the aircraft into heat by absorbing the vibrations. This absorption causes a reduction in noise generated from the inside skin of the aircraft. Typically, damping tiles that absorb vibrations at frequencies above about 500 Hertz provide a reduction in noise from about three decibels to about seven decibels. Fiberglass blankets are placed over these damping tiles to provide absorption of noise. As a result, these fiberglass blankets provide additional reductions in noise.
Trim panels are panels seen inside the cabin. These trim panels are typically placed as another layer after the fiberglass blanket. These trim panels also help reduce noise in the interior of the aircraft. Isolators may be used to mount the trim panels to the structure of the aircraft. These isolators also may reduce noise. In some cases, foam may be used in place of or in addition to the fiberglass blankets to reduce noise. All of these components, however, add weight and cost to the aircraft.
With the increased noise generated by composite structures, thicker materials or additional elements may be used to reduce the noise in the interior of the aircraft. These solutions, however, further increase the weight and cost of the aircraft. Additionally, increased time is needed to manufacture aircraft because of the labor needed to install these components. For example, the process of adding damping tiles and fiberglass blankets is labor intensive in addition to adding undesired weight. Also, damping tiles are expensive and are individually installed on each skin panel.
Therefore, it would be advantageous to have a method and apparatus which takes into account one or more of the issues discussed above, as well as possibly other issues.